In general, complex man-made structures, whether stationary such as buildings and bridges, or mobile such as moving vehicles operating on land, sea, air, or space, are normally made from many components attached together forming a complex structure. The design of attachment points, commonly known as joints, requires special knowledge and skill for engineering design and analysis. A major part of this task is the selection of proper components, such as fasteners, for joining and fastening the structure together.
The main purpose and primary objective in joint design is to facilitate the load transfer from one component of the structure to another component. The joined structure should be able to sustain the external and internal loads that may be experienced during its intended function. Loading may be in sustained static form or in a variable dynamic form. The functioning environment may be corrosive in nature affecting material properties and integrity of the fasteners and structural material. The operating environment may also undergo temperature changes affecting the load carrying characteristics of the joint and fasteners. All these factors should be considered in joint design and fastener selection.
Since man's original venture into building structures and moving vehicles, many types of fasteners have been conceived, developed, and used successfully. However, with an ever advancing civilization the need for continuous improvement is always evident. One common feature in most joint designs is to create holes, or apertures, in the joint components, typically referred to as work pieces, to insert and attach the components to each other by placing a suitable fastener in the matching holes. These fasteners, referred to by many different names and terms, are major contributors for constructing buildings, tools, vehicles, and other important structures comprising the present form of civilization and physical life.
The demand for lightweight, high strength aerospace structural components requires usage of composite materials. Composite materials are composed of two major components: load carrying fibers and the bonding matrix. The load carrying components are made from high strength fibers, such as carbon fibers, and the bonding matrix is normally made from nonmetallic materials, such as epoxy, having much less mechanical strength.
Unlike homogenous metallic structures, the fibrous nature of composite material exhibits non homogenous mechanical properties, thus complicating the process of efficient load transfer at mechanical joints. Consequently, fasteners suitable for joining composite structures are needed. Those skilled in the art of fastening understand that efficient load transfer at mechanically joined structures requires a material exhibiting a certain degree of compliance and resiliency. Metallic structures exhibit resiliency and compliance, but the composite materials, lacking adequate ductility, are brittle in nature and are subject to unpredictable brittle type failure at the joint.
The brittle nature and the lack of resiliency of composite materials will cause non uniform distribution of loads to multiple fasteners installed in a single joint. The installation loads required for installing ordinary fasteners will often generate high levels of compressive stresses around the fastener holes of the structure. These compressive forces, when directly applied on composite structures, cause damage in the form of cracks, delamination, and fiber breakage, which adversely affect the load carrying capability of the structure, specifically around the holes in the structure. A new fastener design which minimized this type of damage is needed.
The issue of proper distribution and sharing of the load between the fasteners and the structural components of the joints having multiple fasteners may be partially achieved by precision drilling of close tolerance holes and implementing a process of perfect hole alignment, such as precision match drilling of the holes. However, this solution is extremely difficult to achieve in practice and is very expensive. This problem can also be resolved by utilizing a hole filling type fastener design. The special challenge is that composite structures do not tolerate hole expansion readily since hole expansion tends to cause delamination and cracks in the structure and around the hole aperture. Therefore, while hole filling is desirable, excessive hole expansion of the structure needs to be avoided.
Consequently, a system of fasteners which alleviates the problems inherent in conventional fasteners as described previously is needed.